Compressor start up is a transient condition consisting of two dynamic phases. The first phase, or crank acceleration, is the transition from rest to running speed. For a successful start of the compressor, i.e. ramp-up from rest to running speed, the torque available from the motor must meet, or exceed, the torque demand. The torque demand consists of the torque due to cylinder pressure and the torque required for acceleration. During the initial crankshaft spin-up, the motor must overcome the peak torque occurring over the entire crankshaft revolution and have enough torque capability remaining to accelerate the crank. Starting with the pressure across the compressor equalized, the torque due to cylinder pressure starts at zero foot-pounds. As the compressor spins up, the torque load increases. However, as the crank speed approaches running speed, the inertia of the compressor running gear and rotor effectively reduce the peak torque variations. When suction cut-off unloading is employed, the crank experiences large peak torque values due to extreme pressure changes in the cylinder. Because the crank is not at full speed, the inertia of the system is not great enough to offset the torque requirements. With a limited power source, this extreme torque requirement can be too great to overcome in high pressure conditions such as those due to high ambient temperature. The second phase encompasses the transition from the point when running speed is achieved to a point when normal system operating pressures are attained. After the compressor reaches running speed, it must pump down the low side of the system, i.e. from the compressor suction to the expansion device.